Engine Inlet Ducts
Normally, the inlet duct of an engine is considered to be part of the airframe instead of the engine itself. Nevertheless, the engine inlet is usually identified as station one and is therefore called Engine Station Number One. When discussing the construction of a gas turbine engine, understanding the function of the inlet duct is rather important since it has a great impact on engine performance.

Purpose of the Inlet Duct
The purpose of the engine inlet duct is to provide the engine compressor with a uniform supply of air in order to prevent the compressor from stalling. Since the inlet is directly exposed to the impacting airflow, it must also create as little drag as possible. It has been found that even the smallest discontinuity of airflow supply can cause major engine problems as well as significant efficiency losses. It is obvious that if the inlet duct is to retain its function of providing sufficient air with a minimum of turbulence, it should be kept clean and close to a condition as if it was new. In some cases it may be necessary to have the inlet repaired which, of course, should be done expertly by installing flush patches in order to prevent additional drag.

Subsonic Inlets
The inlet duct, found on most business and commercial jet aircraft, is of fixed geometry and is divergent of shape. Divergent means that its diameter increases from front to back (figure1). As air enters the aerodynamically contoured shape it starts to diffuse, arriving at the compressor at a slightly higher than ambient pressure. The reason why pressure increases is because volume increases as well, thus velocity should decrease according to the theorem of Bernoulli. In general, the inlet ducts' shape allows the air to diffuse in the front portion of the inlet and to progress at a fairly constant pressure to the compressor section. This causes a much smoother supply of air at a more constant pressure and consequently better engine performance.

Engine Inlet duct shape
Figure 1 - Diverging Inlet Duct

As speed increases inlet pressure increases as well, adding significantly to the mass airflow untill the aircraft reaches its cruising speed. At this point the compressor reached its aerodynamic design point and produces its optimum compression ratio together with best fuel efficiency. This point also marks an important performance factor viewed through the eyes of the engine manufacturer. It is this point where the flight inlet, compressor, combustor, turbine and exhaust are in match with each other. If, for any reason, one of these sections does not match the other sections, engine performance could be effected severly and may even cause damage to the engine.

Earlier on, we've discussed the turbojet and turbofan. In principle, the turbofan inlet is similar to that of the turbojet except that it only discharges a portion of the air through the core engine while the remainder passes into the fan. The turbofan can have multiple airflow arrangements depending on engine size and desired bypass-ratio. Figure 2 displays two common bypass constructions. Figure 2A shows a design which is fully ducted and utilized on low and medium bypass engines while figure 2B shows a short duct design of a high bypass engine. The long duct shown in figure 1A enhances thrust as it reduces surface drag of the fan discharge air.

Turbofan low bypass
Turbofan high bypass
Figure 2A - Turbofan with low bypass ratio
Figure 2B - Turbofan with high bypass ratio

Although you might think that the inlet duct has to feed the needs of the engine only you might want to have a look at the inlet of a CFM-56 installed on the Boeing 737 (Figure 3). Normally the inlet is fairly round but as the ground clearance between the engine and the ground is limited on the 737 the inlet has a somewhat oval shape.

Ram Pressure Recovery
As the engine operates on the ground, the pressure within the inlet has a negative value due to high velocity airflow. After the airplane took off and gained speed, a condition called ram pressure recovery takes place. At this point the pressure within the inlet returns to ambient. Ram compression is therefore a direct result of ram velocity and diffusion of the airflow. The average speed at which ram pressure recovery takes place lies between Mach 0.1 and Mach 0.2.